Method of producing food grade hydrated lime

ABSTRACT

A method is shown for producing food grade hydrated lime. A source of raw, hydrated lime is first passed through a classification step which divides the raw hydrated lime into a first fine stream and a first coarse stream. The first coarse stream is then passed to a grinder which produces a ground coarse product. The first fine stream is separated out from the first coarse stream and, without combining the first fine stream with the first coarse stream or with the ground coarse product, is removed to produce a very fine sized product which meets CODEX chemical specifications.

BACKGROUND OF THE INVENTION

[0001] A. Field of the Invention:

[0002] The present invention relates to a method for producing foodgrade hydrated lime, Ca(OH)₂, which meets the standards as required byCODEX without requiring extensive changes in existing equipment orprocess steps.

[0003] B. Description of the Prior Art

[0004] Lime has a variety of uses. It is commonly used in treating wastewater and sewage. It is used in agriculture to neutralize acidic soilsand to provide nutrients for sustaining plant life. Lime is also usedextensively in construction for the stabilization of soils and as acomponent in a variety of building materials. Lime is also used in avariety of “food grade” products intended for human consumption.

[0005] In this description, the term “lime” will be understood to meanboth quicklime (calcium oxide) and hydrated lime (calcium hydroxide).Quicklime is produced by heating limestone (calcium carbonate) in a kilnat extreme temperatures to “calcine” the material and thereby drive offcarbon dioxide. Quicklime is usually in the form of lumps or pebbles. Inorder to further process lime and improve the ease with which it ishandled, dry lime is often mixed with water to form a slurry. In thecase of quicklime, the water reacts with the quicklime in an exothermicreaction to form hydrated lime. This is often referred to as slaking.During the slaking of quicklime, large amounts of heat are given offwhich can significantly raise the temperature of the slurry. Water canthen be driven off to produce dry, hydrated lime which is usually apowder.

[0006] Food grade hydrated lime is a specific material that is sold tothe food processing industry in the United States. The specificationsfor food grade hydrated lime are defined by CODEX. The CODEXAlimentarius Commission was created in 1962 by two U.N. organizations,the Food and Agricultural Organization (FAO) and the World HealthOrganization (WHO). CODEX is the major international mechanism forencouraging fair international trade in food while promoting the healthand economic interests of consumers. Within the United States, CODEXactivities are coordinated by officials from the U.S. Department ofAgriculture, the U.S. Food and Drug Administration and the U.S.Environmental Protection Agency. In the United States, there are anumber of companies that produce “normal”, i.e., industrial grade,hydrated lime for industrial use. However, there are only two companiesknown to Applicant at the present time that produce “food grade”hydrated lime. This is due, at least in part, to the exacting chemicalspecifications required by CODEX. Many hydrated limes that are suitablefor general industrial use, fail to meet the CODEX standards since theyexceed, for example, the limits for trace metals found in thecompositions.

[0007] As mentioned above, hydrated lime is produced by first heatinglimestone in a kiln (calciner) to remove carbon dioxide and formquicklime. To improve the handling characteristics of the quicklime, acontrolled amount of water is then added to form calcium hydroxide,commonly referred to as hydrated lime. To achieve the CODEX chemicalspecifications requires the use of the purest limestone as calciner feedand normally the use of natural gas as a fuel for the kiln. The use ofnatural gas, even though it is more expensive, is preferred over solidfuel such as coal or coke, in that solid fuel tends to contaminate thequicklime and thus the hydrated lime made from the quicklime withcombustion ash.

[0008] A need exists for a method for producing food grade hydrated limewhich meets CODEX specifications without drastic changes in theequipment or process parameters presently employed in calcininglimestone to produce quicklime, or in slaking the quicklime to producehydrated lime.

[0009] A need exists for a method for producing food grade hydrated limewhich would allow the use of normal limestone as feed to the calciner,without requiring the purest of limestone as feed.

[0010] A need also exists for such a process which would allow the useof solid fuel sources in the calciner, rather than requiring the use ofmore expensive natural gas as a fuel source.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a method forproducing food grade hydrated lime without requiring unusually purelimestone as calciner feed.

[0012] It is also an object of the invention to provide a method forproducing food grade hydrated lime which allows the use of typical solidfuels to fuel the calciner, rather than requiring the use of moreexpensive natural gas as a calciner fuel.

[0013] It is also an object of the present invention to provide aprocess for manufacturing food grade hydrated lime which separates butretains the larger coarse particles of hydrated lime, as well as thefiner, screened particles, so that the larger sized particles can beprocessed as normal hydrated lime with no waste being introduced intothe process.

[0014] The present invention concerns an alternative method of producingfood grade hydrated lime without the stringent control of limestone feedand calciner fuel. It has been discovered that some “normal” limestonecalciner feed has its impurities distributed over different particlesizes. Additionally, the combustion ash deposited on the quicklime (CaO)also varies in particle size. During the hydration reaction thatconverts calcium oxide to calcium hydroxide, these impurities can beseparated and removed from the calcium hydroxide particles. In this way,a food grade hydrated lime can be produced from quicklime (CaO) thatpreviously did not meet the CODEX chemical specifications.

[0015] In the method of the invention, a source of raw, hydrated lime isfirst passed through a classification step which divides the rawhydrated lime into a first fine stream and a first coarse stream. Thefirst coarse stream is then either recycled in the process or passed toa grinder which produces a ground coarse product. The first fine streamis separated out, without combining the first fine stream with the firstcoarse stream or with the ground coarse product, and is removed toproduce a very fine sized product which meets CODEX chemicalspecifications. While the method of the invention can be practiced in aone step separation process, the method is preferably practiced in a twostep separation process. The source of raw, hydrated lime is firstpassed through a classification step as before to produce a first coarsestream having particle sizes in the range from about +10 to 50 mesh orlarger. The first coarse stream is passed to a grinder which produces aground coarse product. A first fine stream which is produced in theprimary classification step is subjected to a secondary classificationstep and very fine product is separated out, without combining the veryfine stream with the first coarse stream or with the ground coarseproduct, to produce a very fine sized product having a particle size ofabout −325 mesh or finer which meets CODEX chemical specifications.

[0016] Additional objects, features and advantages will be apparent inthe written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a simplified, schematic of a prior art process forproducing industrial grade hydrated lime; and

[0018]FIG. 2 is a schematic view, similar to FIG. 1, showing the methodsteps of the method of the present invention which is used to producefood grade hydrated lime.

[0019]FIG. 3 is a schematic view, similar to FIG. 2 but showing analternative screening method of the invention in which a slurry is usedto separate out coarse particles.

[0020]FIG. 4 is a schematic view, similar to FIG. 3 showing anothermethod of using a slurry to screen and separate out coarse particles.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In the discussion which follows, the term “quicklime” will betaken to mean calcium oxide and should not be confused with limestone(calcium carbonate). As briefly outlined in Applicant's backgrounddiscussion, quicklime is manufactured from limestone by heating toremove carbon dioxide. Quicklime can be converted to Ca(OH)₂ by aslaking process where water and CaO are mixed under agitation andtemperature to produce Ca(OH)₂, known in the industry as slaked lime orlime hydrate.

[0022]FIG. 1 is a simplified, schematic diagram which illustrates theprior art process for producing industrial grade hydrated lime. Rawlimestone is first fed to a calciner 11 which is typically a horizontalor vertical kiln. The kiln is fired by burners which typically utilizepulverized coal as a fuel and are capable of reaching calciningtemperatures in excess of 1600° F. The intense heat causes a chemicalreaction as follows:

CaCO₃+heat=CaO(quicklime)+CO₂

[0023] The quicklime produced in the calciner 11 is then slaked bymixing with an aqueous slaking medium in hydrator 13. This results in anexothermic reaction generating heat and calcium hydroxide:

CaO+H₂O=Ca(OH)₂+heat+steam

[0024] The size and quality of slaked lime particles in the resultingslurry are dependent on a number of variables. These include thereactivity, particle size and gradation of the quicklime used. Othervariables include the amount of water used, the quality of the water,and the amount and type of water impurities. Further, the temperature ofthe water and the amount of agitation can affect slaked lime quality andparticle size.

[0025] The excess water not converted to calcium hydroxide is heated tosteam and the steam is volatized from the solid calcium hydroxideparticles. The solid calcium hydroxide leaving the reactor is composedof individual calcium hydroxide particles, agglomerated calciumhydroxide particles, individual impurity particles, and impuritiesassociated with the individual and agglomerated calcium hydroxideparticles. These materials are represented as “dry agglomerate” in thestep 15 shown in FIG. 1 and comprise a broad distribution of calciumhydroxide particles.

[0026] This broad distribution of particles is then normally screened orair classified in a step 17 which divides the raw hydrated lime into afirst fine stream 19 and a first coarse stream 21. Because a largepercentage of the calcium hydroxide is present in the coarse fraction ofstream 21, this fraction is normally ground in a dry grinding step 23and then returned to the fine calcium hydroxide particles in a stream25, the particles being mixed in a step labeled as 27 in FIG. 1. Duringthis process all the impurities present in the quicklime, calcium oxidefeed are also present in the final calcium hydroxide product whichpasses out stream 29.

[0027] Applicants have discovered that for most quicklime/hydrated limeproductions, the impurities that are important for CODEX specificationsare uniformly distributed in all particle sizes of the calcium hydroxidewhich is formed according to the above described prior art process.However, Applicants have also discovered that certain sources of rawfeed materials (calcium carbonate) allow the production of food gradehydrated lime which meets the CODEX impurity specifications byincorporating an additional process step which removes the large coarserparticles that are formed during the hydration reaction.

[0028] The specific process is to take the “raw” calcium hydroxide fromthe reactor and then screen or air classify the material to remove thecoarse +10 to 100, most preferably +10 to 50, mesh particles. The finefraction can then be further screened or air classified to −325 mesh toachieve the necessary purity. While the preferred method of theinvention is a two step separation process to achieve higher yields offiner product, it is within the scope of the invention to have a single(one size) separation to meet the CODEX specifications. The criticalpoint in the method of the invention is to insure that the coarsefraction of calcium hydroxide is not processed as in the prior art, bygrinding and adding it to the fine food grade calcium hydroxide.

[0029] The improved method for producing food grade hydrated lime isillustrated in FIG. 2 of the drawings. Raw limestone is calcined in astep 31, as before. The resulting quicklime is hydrated in a step 33with the dry agglomerate of step 35 being fed to an air classifier in astep 37. The separated out coarse particles in stream 39 pass to a drygrinder 41 where they are reduced in size and fed through stream 43 tomixing point 45.

[0030] In the preferred method of FIG. 2, the first fine stream ofparticles 47 are passed to a secondary separation step in air classifier49. A resulting relatively coarse stream 51 can be returned to themixing point 45 to be combined with the product in stream 43, therebyproducing old product in outlet stream 53. In the improvement of theinvention, the very fine particles (−325 mesh) in stream 55 remainseparated from the second coarse stream 51 and are removed as productthrough outlet stream 57 without combining the first coarse stream 39 orwith the ground coarse product 43. The product removed through outletstream 57 is a very fine sized product which meets CODEX chemicalspecifications.

[0031] The invention is thus intended to encompass a two step separationprocess in which the coarse calcium hydroxide particles from the firstseparation, which contain a larger portion of the impurities, can beground and added to the coarse fraction of the second size separation(shown being combined in the mixer 45 in FIG. 2). This product can meetthe specification for “normal” hydrated lime with no waste and 100% ofthe quicklime/hydrated lime being utilized in the process. The dottedline 59 in FIG. 2 is intended to represent the option of passing aportion of the first fine stream to old product, rather than furtherprocessing it to produce food grade product, at the option of theoperator.

[0032] The following Table I shows the results of seven differentquicklime/hydrated lime systems and how the impurities are affected byparticle size separation. Note that no sample achieved CODEX food gradespecifications with the then currently available hydrated lime. Onlysample #7 achieved CODEX food grade specification with impuritiesremoved in a coarse fraction. TABLE I Sample # 1 Sample # 2 Sample # 3Sample # 4 Sample # 5 Sample # 6 Sample # 7 −100 −100 −100 −100 −100−100 −100 FCC specifications HL mesh HL mesh HL mesh HL mesh HL mesh HLmesh HL mesh Calcium hyd. 95.0% min. 96.5 97 96.1 95.7 96.8 95.1 95.595.7 93.4 95.2 96.1 96.3 97.1 97.2 Acid insoluble 0.50% max 2 0.4 0.30.1 0.2 0.1 1.8 1.5 0.3 0.2 1.9 0.7 1.0 0.2 Magnesium  4.8% max 0.5 0.20.35 0.3 0.2 0.3 1.0 0.9 1.7 1.9 1.3 1.0 0.3 0.2 Arsenic  3 ppm max0.9-20.0 0.02 0.1 4.0 3.0 1.3 0.6 Fluoride 50 ppm max 45 64 65 80 100 5636 Lead 10 ppm max 0.63 0.8 0.5 0.7 0.8 0.37 0.44 Heavy metals 30 ppmmax 60 <30 <30 <30 <30 60 <30

[0033] The invention has been described with respect to an airclassifier separation step, followed by dry grinding of coarse productin one preferred embodiment of the invention. Thus skilled in the artwill also appreciate that the separation of coarse from fine particlescould also be accomplished by using a slurry to separate out the coarseparticles. In FIG. 3, quicklime from the calciner 69 passes through ahydration step 71 as in the previously described method. In this case,however, the raw hydrated lime is slurried by the addition of an aqueousmedium in a step 73 and the slurry is then screened to separate out the+100 to +325 mesh coarse particles in a stream 75 with the fineparticles passing out in a separate stream 77.

[0034] In FIG. 4, quicklime from the calciner 79 is slaked in a step 81to produce calcium hydroxide. The slaked calcium hydroxide is thenscreen, as before, to separate out the +100 to +325 mesh coarseparticles in a stream 83 with the fine particles in the slurry passingout the separate stream 85.

[0035] An invention has been provided with several advantages. Themethod of the invention allows food grade hydrated lime to be producedwith little change in existing process equipment or processing steps.The method is simple in design and economical to implement. In additionto producing a new food grade product stream, the process can utilizethe separated coarser hydrate particles by combining these particleswith an old product stream to produce an industrial grade hydrated lime.

[0036] While the invention has been shown in only two of its forms, itis not thus limited but is susceptible to various changes andmodifications without departing from the spirit thereof.

I claim:
 1. A method for producing high purity, food grade hydratedlime, the method comprising the steps of: providing a source of limeselected from the group consisting of raw hydrated lime and quicklime;forming the source of lime into a liquid slurry by either combining theraw hydrated lime with an aqueous medium or by slaking the quicklime,followed by passing the slurry through a classification step in whichthe slurry is screened into a first fine stream and a first coarsestream having a particle size range from about +100 to +325 mesh;separating out the first fine stream and, without combining the firstfine stream with the first coarse stream, removing the first fine streamto produce a product which meets CODEX chemical specifications.
 2. Amethod for producing high purity, food grade hydrated lime, the methodcomprising the steps of: providing a source of raw hydrated lime;passing the raw hydrated lime through a classification step whichdivides the raw hydrated lime into a first fine stream and a firstcoarse stream; passing the first coarse stream to a grinder whichproduces a ground coarse product; separating out the first fine streamand, without combining the first fine stream with the first coarsestream or with the ground coarse product, removing the first fine streamto produce a product which meets CODEX chemical specifications.
 3. Themethod of claim 2, wherein the classification step is accomplished bymeans of an air classifier.
 4. The method of claim 2, wherein the firstcoarse stream is comprised of particles having a particle size rangefrom about +10 to 100 mesh.
 5. The method of claim 4, wherein the firstfine stream is subjected to a further grinding step to produce a veryfine product having a particle size in the range of about −325 mesh andbelow in one stream and a second coarse stream.
 6. The method of claim5, wherein the first coarse stream, containing a larger portion ofimpurities, is added to the second coarse stream to form a less pureproduct.
 7. A method for producing high purity, food grade hydratedlime, the method comprising the steps of: providing a source of rawhydrated lime; passing the raw hydrated lime through a classificationstep which divides the raw hydrated lime into a first fine stream and afirst coarse stream having particle sizes in the range from about +10 to50 mesh; passing the first coarse stream to a grinder which produces aground coarse product; separating out the first fine stream and, withoutcombining the first fine stream with the first coarse stream or with theground coarse product, further grinding the first fine stream to producea very fine product stream and removing the very fine stream to producea very fine sized product having a particle size of about −325 mesh orbelow which meets CODEX chemical specifications.
 8. The method of claim7, wherein the classification step is accomplished by means of an airclassifier.
 9. The method of claim 8, wherein the raw, hydrated limefeed is prepared by slaking quicklime with water.
 10. The method ofclaim 9, wherein the first coarse stream, containing a larger portion ofimpurities, is added to the second coarse stream to form a less pureproduct in addition to the very fine product which is produced.